The present invention relates in general an analog to digital (A/D) converters. More specifically, the invention provides an A/D converter suitable for use in digital video apparatus such as, for examples, digital television sets, digital video tape recorders or the like.
With the recent development of digital video apparatus, there is demand for a high-speed and high resolution A/D converter which can be manufactured as a monolithic integrated circuit (IC).
A conventional converter, so called a "full parallel converter", as disclosed in J. G. Peterson, "A Monolithic Video A/D Converter", in IEEE JOURNAL SOLID-STATE CIRCUITS, VOL. SC-14 No. 6, DECEMBER 1979, pp. 932-937, provides the best speed performance among known converters. However, a drawback of this type of converter is that it requires a great number of comparators (2.sup.n -1) to obtain an n-bit binary coded parallel output signal. For example, an 8-bit converter requires 255 comparators. Furthermore, the number of elements used, especially the number of transistors, is large. Therefore, the full parallel converter is not suitable for monolithic integration.
On the other hand, as disclosed in R. J. VAN DE PLASSCHE et al "A High-Speed 7 Bit A/D Converter" in IEEE JOURNAL OF SOLID-STATE, VOL. SC-14. NO. 6, DECEMBER 1979, pp. 938-943, a two-step system converter is known in which a sample-and-hold circuit is used together with a high-speed A/D-D/A coarse quantizer subtractor circuit.
This type of converter includes a first-stage comparator A/D converter for providing the most significant bits of digital output and a second-stage comparator A/D converter for providing the least significant bits of the digital output.
In the first-stage converter first voltage comparators compare an analog input signal level to be converted into digital form with a plurality of first reference voltage levels. A first decoder provides the most significant bits in response to the comparison result. The most significant bits are converted by a digital-to-analog (D/A) converter into an analog voltage. The difference between the converted analog voltage and the analog input signal is obtained.
In the second stage converter, the analog voltage difference is compared by second voltage comparators with a plurality of second reference voltage levels. A second decoder produces the least significant bits on the basis of the comparison result. Such a two-step converter can be made with a reduced number of comparators vis-a-vis a full parallel converter.
However, the conversion speed of this type of converter is lower than that of the full parallel converter and it is not suitable for use at video frequencies. Furthermore, it is not suitable for monolithic integration.